The present invention relates to a high-speed rotor and, particularly, to a controller for controlling magnetic bearings that are desirably used for a supercritical rotor.
Conventional radial magnetic bearings use electromagnets that are arranged in opposing relationship to each other. By this arrangement, a rotor is attracted and is supported by magnetic bearings in a contactless manner. In order to effectively utilize the attractive force, the rotor is usually fitted with magnetic steel sheets and forms a magnetic loop together with a laminated core of the electromagnet. In order to stably support the rotor, the position of the rotor is monitored at all times using a displacement sensor and the electromagnetic force must be so controlled according to the output signals of the displacement sensor that the rotor is stably supported. A control method called PID control is usually employed for a compensator for the magnetic bearing. There has been employed a control system which eliminates any deviation of the rotor from the center of the gap of the bearing. A system for controlling the magnetic bearing is disclosed in "Magnetic Levitation and Magnetic Bearing, Corona Co., Jun. 30, 1993, pp. 184-192" and "Active Magnetic Bearings, vdf Hoschschulverlag AG an der ETH Zuerich, 1994".
The bearing stiffness of a magnetic bearing is usually one-tenth smaller than the bearing stiffness of oil bearings. In a conventional machine using oil bearings' such as a centrifugal compressor or the like machine, therefore, the range of operation can be designed to lie between first and second critical speeds. By using the magnetic bearings, however, it is unavoidable to design the range of operation to lie between the 3rd and 4th critical speeds. The two modes from the lowest critical speed side are rigid modes of the rotor, and the 3rd and 4th critical speeds are bending modes of the rotor. There will be no problem in operation, if imbalanced vibration is suppressed to a sufficient degree and the range of operation is ensured despite an increase in the critical speed that results from a low bearing stiffness and that must hence be exceeded. Generally, however, the distances between critical speeds become smaller than the range of operation as the order becomes higher, which makes it difficult to design the rotor and the control circuit.
In order to cope with the vibration displacement of the rotor in the bending mode, Japanese Patent Laid-Open No. 280542/1993 discloses a method in which a displacement sensor is disposed near the electromagnet, and the action point of the electromagnet and the point of position detection are brought into agreement in order to improve the controllability. Though the method disclosed in this Japanese Patent Laid-Open No. 280542/1993 tries to improve the controllability of the bending mode by optimizing the arrangement of the displacement sensor, no consideration is given to the imbalance of the force, which is a major cause of bending vibration, thereby still leaving, a large bending displacement.